kdbus0 is a replacement for
the D-Bus daemon which is used in GNU/Linux and Unix systems, originally
authored by Lennart Poettering of Red Hat. It represents a monumental
increase in complexity, a slap in the face to the Unix philosophy, and
its inherent domineering and viral nature turns it into something akin
to an "in-kernel" implementation of D-Bus that is spreading all across the Linux ecosystem. This
site aims to serve as a rundown and a wake-up call to take a stand
against the widespread proliferation
of kdbus, to detail why it is harmful, and to persuade users to
reject its use, and especially its
ubiquity.

Disclaimer: We are not D-Bus purists by any means. We do recognize the need for a new D-Bus system in the 21st century, but
kdbus is not it.

The Rundown

1. kdbus flies in the face of the Unix
philosophy: "do one thing and do it well," representing a complex
collection of dozens of tightly coupled binaries1. Its
responsibilities grossly exceed that of an IPC system, as it interfears with
power management, device management, mount points,
cron, disk encryption, socket API/inetd, syslog, network
configuration, login/session management, readahead, GPT partition
discovery, container registration, hostname/locale/time
management, mDNS/DNS-SD, the Linux console and other things ....probably. The agenda for kdbus to be an ever-growing and
invasive message oriented meddleware for GNU/Linux was elucidated in a 2014 Auckland Linux Conf talk
2. Keep it simple, stupid.

2. kdbus's journal files (handled by journald) are stored in a
complicated binary format3, and must be
queried using journalctl. This makes journal logs potentially
corruptible, as they do not have ACID-
compliant transactions. You typically don't want that to happen to
your syslogs. The advice of the kdbus developers?
Ignore
it.
The only way to generate traditional logs is to run a standard syslogd
like rsyslog alongside the journal4.
There's also embedded HTTP server integration (libmicrohttpd). QR codes are served, as well, through libqrencode.

3. Since kdbus is very tightly welded with the
Linux kernel API, different kdbus versions are incompatible with
different kernel versions and portability is unnecessarily
hampered in many components. This is an isolationist policy that
essentially binds the Linux ecosystem into its own cage, serving
as an obstacle to developing software portable with both Linux
variations and other Unix-like systems. It also raises some issues
backporting patches and maintaining long-term stable systems.

4. udev and dbus are forced dependencies. In fact, udev merged
with kdbus a long time ago5. The integration of the device node
manager, which was once a part of the Linux kernel, is not a
decision that is to be taken lightly. The political implications
of it are high, and it makes a lot of packages dependent on udev,
in turn dependent on kdbus, despite the existence of forks, such
as eudev. Starting with kdbus-209, the developers now have their
own, non-standard and sparsely documented sd-bus API that replaces
much of libdbus's job, and further decreases transparency.
Further, they intend to migrate udev to this new transport,
replacing Netlink and thus making udev a kdbus-only
daemon6. The
effects of this move are profound.

5. kdbus features a helper which captures coredumps and directs
them either to /var/lib/kdbus/coredump... or the journal, where
they must be queried using coredumpctl7. The latter behavior was a
default and is likely to return8. It assumes that users and admins
are dumb9, but more critically, the fundamentally corruptible
nature of journal logs makes this a severe impediment, and an
irresponsible design choice. It can also create complications in
multi-user environments related to privileges.

6. kdbus makes the kernel a single point of
failure. As of this writing, kdbus has had some CVE reports, since
its inception in March 201010. So far, this may not seem like that
much, but its essential and overbearing nature will make it a
juicy target for crackers, as it is far smaller in breadth than
the Linux kernel itself, yet seemingly just as critical.

7. kdbus is viral by its very nature, due to its auxiliaries
exposing APIs, while being bound to kdbus's init. Its scope in
functionality and creeping in as a dependency to lots of packages
means that distro maintainers will have to necessitate a
conversion, or suffer a drift. As an example, the GNOME
environment often makes use of kdbus components, such as logind,
and support for non-kdbus systems is becoming increasingly
difficult. Under Wayland, GNOME relies on logind, which in turn
requires and is a part of kdbus11. More and more maintainers are
going to require kdbus for this reason, and similar instances
like it. The rapid rise in adoption by distros such as Debian,
Arch Linux, Ubuntu, Fedora, openSUSE and others shows that many
are jumping onto the bandwagon, with or without justification.
Other dependent packages include the Weston compositor, Polkit,
upower, udisks2, PackageKit, etc. It's also worth noting that
kdbus will refuse to start as a user instance, unless the system
boots with it as well - blatant coercion12.

8. kdbus clusters itself into PID 1, rather than acting as a
standalone process supervisor. Due to it controlling lots of
different components, there are tons of scenarios in which it
can crash and bring down the whole system. We should also
mention that in order to reduce the need for rebooting, kdbus
provides a mechanism to reserialize and reexecute systemctl in
real time, however, if this fails, of course, the system goes
down. There are several ways that this can occur13, including an
inability to reload a previous, potentially incompatible state.
This happens to be another example of SPOF and an unnecessary
burden on an already critical component (init).

9. kdbus is designed with glibc in mind, and
doesn't take kindly to supporting other libcs all that
much14. In
general, the kdbus developers' idea of a standard libc is one
that has bug-for-bug compatibility with glibc.

10. kdbus's complicated nature makes it harder to extend and
step outside its boundaries. While you can more or less trivially
start shell scripts from unit files, it's more difficult to write
behavior that goes outside the box, what with all the feature
bloat. Many users will likely need to write more complicated
programs that directly interact with the kdbus API, or even
patch kdbus directly. One also needs to worry about a much
higher multitude of code paths and behaviors in a system-critical
program, including the possibility of kdbus not synchronizing
with the message bus queue on boot, and thus freezing. This is as
opposed to a conventional init, which is deterministic and
predictable in nature, mostly just serially execing scripts.

11. Ultimately, kdbus's spread is symbolic of something more
than kdbus itself. It shows a radical shift in thinking by the
Linux community. Not necessarily a positive one, either. One that
is heavily desktop-oriented, choice-limiting, isolationist,
reinvents the flat tire, and is just a huge anti-pattern in
general. If your goal is to pander to the lowest common
denominator, so be it. We will look for alternatives, however.

12. kdbus doesn't even know what it wants to be. It is
variously referred to as a "system daemon" or a "basic userspace
building block to make an OS from", both of which are highly
ambiguous. It engulfs functionality that variously belonged to
util-linux, wireless tools, syslog and other projects. It has no
clear direction, other than the whims of the developers
themselves. Ironically, despite aiming to standardize Linux
distributions, it itself has no clear standard, and is perpetually
rolling.